1. Field of the Invention
The present invention relates to a tractor frame structure, and further to a manner for producing the tractor frame.
2. Background Art
A small tractor requires a simple tractor frame which is economical but strong enough to endure the weight of an engine mounted thereon. Therefore, a typical conventional simple tractor frame which is adaptable for a small tractor is longitudinally straight when viewed from the side, wherein left and right parallel side plates having a whole length of the tractor are extended straight in the longitudinal direction of the tractor.
Furthermore, a small tractor, if possible, preferably places its footboards for resting respective driver's feet thereon low enough to facilitate the driver's getting on and off the tractor. However, even if the tractor is of a small type, a front portion of the tractor frame must be high enough to be provided therebelow with a front axle casing. If the footboards are placed onto a longitudinally intermediate portion of the above-mentioned longitudinally straight tractor frame, the footboards become rather too high for getting on and off, thereby being inconvenient to such a small tractor that requires its footboards to also serve as steps for getting on and off the tractor.
Therefore, conventionally, as disclosed in Japanese Utility Model Laid Open Gazette No. Sho 57-15768 there has been provided a well-known tractor having a (cranked) tractor frame bent at its longitudinally intermediate portion so as to locate its rear level portion, over which footboards are spread, lower than its front level portion supporting the front axle casing and so on. However, each of left and right side members of the disclosed tractor frame is made of a bent plate having a constant width in its whole length, whose strength is poor at the bent longitudinally intermediate portion.
Furthermore, if a front working attachment such as a front loader is to be attached onto a front portion of the tractor, the conventional tractor frame is provided below the longitudinally intermediate portion thereof with a support frame for supporting the front working attachment so as to reinforce the tractor frame, thereby resulting in a troublesome work. It is still more complicated to provide the front working attachment onto a mid-mount tractor provided below the longitudinally intermediate bottom portion thereof with a middle working attachment such as a mower, because the middle working attachment must be removed so as to be prevented from interfering with the front working attachment or the front working attachment support frame. On the other hand, there arises a problem in the poor strength of the tractor frame if the front working attachment support frame is attached to lateral sides of the above-mentioned cranked side members so as to be prevented from interfering with the middle working attachment.